Check this location above and behind the rear bumper, on both sides mine was marred/scratched, missing paint, and appeared to be a fixture mounting location. It's the "black" area on the body in the below photo's that I touched up with black chassis paint, unfortunately I didn't take before photos. Mine may have slid on the fixture, not sure if it would have rusted or corroded but I know it won't now. I have not found any other locations on the underbody missing any paint.
